feat: update Slack plugin to HTTP transport with OAuth config

- Switch from SSE to HTTP transport
- Update URL from /sse to /mcp endpoint
- Add OAuth clientId and callbackPort for Claude Code auth flow

external_plugins/slack/.mcp.json

@@ -1,6 +1,10 @@
 {
   "slack": {
-    "type": "sse",
-    "url": "https://mcp.slack.com/sse"
+    "type": "http",
+    "url": "https://mcp.slack.com/mcp",
+    "oauth": {
+      "clientId": "1601185624273.8899143856786",
+      "callbackPort": 3118
+    }
   }
 }